The present invention relates to a longitudinally adjustable bow for glasses with a first bow section having the shape of an essentially closed hollow section, and a second bow section slidably guided in the first bow section, and with a stop device consisting of a plurality of stop notches, at least one catch and at least one flexibly pre-stressed area.
In such a longitudinally adjustable bow, known from German Published, Examined Patent Application DE-AS 22 12 531, the catch is provided on the movable second bow section and the stop notches are disposed on the underside of the hollow section of the first bow section. By means of the flexibly pre-stressed area of the second bow section provided with the catch, the catch reaches the opposite stop notch in the first bow section. In this case the longitudinal adjustment takes place in such a way that the first bow section is held fast and pushing or pulling is performed on the second bow section in order to pull or push in this way the catch, which is seated in a stop notch, out of the latter and to push it into the next stop notch. Because such removal from a certain stop position must be performed exclusively by use of force, the danger of damage or bending of the bow sections is relatively great if it is taken into consideration that the user himself wants to make such a longitudinal adjustment. It is furthermore conceivable that with this type of forceful release of the stop device the locking power wanes with time and it is possible that the bow can shift accidentally.
A longitudinally adjustable bow operating in this way is also known from U.S. Pat. No. 4,544,245 where, although the stop notches are disposed in the movable second bow section and the catch inside the hollow section of the first bow section, the second bow section is guided without perceptible play in the first bow section. However, because the second bow section does not have a flexibly pre-stressed area, the catch and/or the stop notches themselves must be deformable on the second bow section with respect to the first bow section by the application of pull or push in order to reach another stop position. This known longitudinally adjustable bow therefore has the above mentioned disadvantages to a higher degree.
Furthermore, a longitudinally adjustable bow is known form German Utility Model DE-GM 85 07 200, the first bow section of which is made of a flat material and is provided on the outside of both parallel longitudinal edges with stop notches and on its free end with a guide eye for the second bow section. The second bow section, which also is made at least partially of a flat material is provided on its end extending over the first bow section with a slitted, sleeve-like eye which overlaps the first bow section in a pre-stressed manner in such a way that it can catch in the stop notches of the first bow section. Although the slitted, sleevelike eye can be widened from the outside with the aid of a screwdriver so as to be able to set another stop position without considerable use of force, there are nevertheless problems in regard to possible wearing off of the galvanized surface in the visible part of the first bow section. Furthermore, some skill is needed to make such an adjustment if it is desired to prevent rubbing of the second bow section on the first bow section in the course of the entire adjustment operation.